The initial impetus to the work here described came from the military requirement to communicate over the MILSTAR system, whilst moving, in a ground vehicle, over rough terrain. The scheme is referred to as ‘Comm On The Move’ or simply COTM. The MILSTAR system employs a set of geo-stationary satellites and the ground terminals require dish antennas of sufficient size to concentrate the 3 dB signal beam-width within less than 1°. To successfully communicate over this system, it is essential that the antenna be pointed at the satellite at all times and not be permitted to deviate by more than 0.5° from bore-sight. Since the antenna will be mounted on a platform connected to the vehicle it would be impossible to communicate over the link unless a stabilization system was employed which detects and adequately cancels the vehicle movement. Such a system would be composed of two parts:
1. The accurate and timely estimation of the antenna platform attitude with respect to some convenient frame of reference, and
2. A mechanical assembly comprising servo systems and actuators which can use the gathered attitude data to stabilize the antenna platform. The system to be described is an attempt at a solution to the first part.
The Known State of the Prior Art
The basic concept underlying GPS based attitude angle determination is that of interferometry, which basically measures some attribute of signals received at two (or more) points separated a know distance apart. These attributes and their differences contain information about the inclination of the plane on which the signals were gathered. In the case of GPS interferometry the relevant attributes are the phases of the carrier wave received at a set of antennae.
Various means of implementing GPS interferometry have been proposed over the last 20 years or more. Numerous patents exist each claiming some improvement to the state of the art. But as far as is known, all prior art on the subject have a common feature. They are all based on the assumption that the basic vehicle to be used is the conventional GPS receiver, a functional block of which is shown in FIG. 1.
Referring now to this Figure, the Front End 2 amplifies, filters and down-converts the signal received by the antenna 1 producing the output Sx which is a superposition of signals from all GPS satellites in view. Quadrature Correlation and Code Stripping further reduces the frequency, and then a selection, under computer control, of a particular satellite PN code, permits a search for the proper code phasing and when that is found a simple multiplying action strips that code from the signal. This process is frequently referred to as Initial Acquisition. This process also enhances the signal to be at least 20 dB stronger than any of the un-stripped signals. The process is repeated for all possible satellites.
The outputs of the Quadrature Correlation and Code Stripping process, Cs(j) are the carrier signals for all the j satellites in view. These are next processed in a Phase Locked Loop (PLL) 4 to keep the carrier phase in track, a Frequency Locked Loop (FLL) 5 to stay on the correct frequency and a Delay Locked Loop (DLL) 6 to keep the PN code phasing properly aligned. The PLL 4 also reads the navigation data which is bi-phase modulated on the carrier phase.
The outputs from all of these feedback loops, plus other relevant parameters are passed on to a computer, which processes the data, generates the required outputs and provides controls for the rest of the system.
The use of the three feedback loops, enclosed in dotted lines, imposes certain restrictions on the receiver performance. A closed loop tracking system must have a finite bandwidth, which implies a finite reaction time, usually referred to as ‘settling time’, which is inversely proportional to the loop bandwidth. As that bandwidth increases, the settling time shortens. Another factor crucially affecting settling time is the type and order of the frequency selectivity of the loop. The usual choices here are second order loops and Butterworth frequency responses.
The required settling time can only be meaningfully defined when the required final output accuracy is specified. A phase locked loop's settling time for the output to be within 0.1° of the true value will be substantially longer than that to within say 10°, everything else being the same.
From the settling time perspective, the larger the bandwidth used the better. Unfortunately, as bandwidth increases so does the amount of noise entering the system. A point can be quickly reached where the noise is so excessive that no useful information can be extracted from the available data. Therefore noise considerations and settling times must be balanced to reach some optimal compromise. No general value can be given for optimal bandwidths, because that depends on system requirements, the operational environment and final goals. For example Babitch (U.S. Pat. No. 5,347,286) states that for his application a 1 KHz loop bandwidth is the largest that can be tolerated. Figures of that order of magnitude would realistically apply to most prior art systems.
A 1 KHz, conventionally designed, phase locked loop with output accuracy of the order of 1° of phase, would typically settle in tens, or possibly hundreds of milliseconds. Similar considerations hold for all of the other feedback loops employed.
The prior art receiver's main function is to measure the transit time of the signal from at least four satellites and to translate these transit times into a range measurement, as depicted in FIG. 2.
The coordinate system usually used in GPS work is the Earth Centered Earth Fixed (ECEF) system. This defines the X-axis as the line joining the earth center and the intersection of the Greenwich meridian and the equatorial plane, the Z-axis as the line from the earth's center to the North Pole and the Y-axis as the direction orthogonal to the other two axes depicted in FIG. 2.
Still referring to FIG. 2, once the four ranges R1, R2, R3, R4 have been measured then, since the exact position of the satellites is known, the position of the GPS receiver can be determined by triangulation. However these ranges can also be used for interferometric processing. If the receiver is capable of collecting range measurements at two or more antennae, then the difference in the range measurements as seen by the different antennae will contain information about the inclination of the platform on which the antennae are mounted. This is the embodiment of the well-known prior-art interferometric principle, as applied to GPS receivers.
However, the scheme can only be usefully applied to attitude determination if it can be shown that the ranges can be established with adequate accuracy. This accuracy issue may be understood by next referring to FIG. 3.
The two antennae A and B in FIG. 3 are a distance D apart and the range from the satellite to A and B must be measured with sufficient accuracy to ensure that the angle α can be determined to within a deviation of no more than δ°. Then it follows on inspection of FIG. 3 that d must be less than D·tan(δ). This will define the required range accuracy. As an example let us assume that δ=0.5° and the antenna separation is 1 meter. Then the value of d must be less than 8.7 millimeters. That is an extreme accuracy, considering that the satellite to observer distance is about 20,000 km.
The principle used here to achieve this is as follows: Consider an ideal system, no noise, no losses and everything is stationary. Then the distance of the antenna from the satellite can be expressed as an integer multiple of wavelengths of the transmitted signal plus a fraction of a whole cycle multiplied by the wavelength. If the phase measured at the antenna is φa, then the distance between the antenna and the satellite will be (φa/360)·λ+N·λ. The wavelength λ is known precisely, N will be an integer, which once determined, will be completely precise, so the only noise in this measurement will come from the noise imposed on the phase φa. For a satellite carrier frequency of 1575.42 MHz, a 1 mm range precision implies phase knowledge to within 1.89°, which is not very difficult to achieve. The only problem remaining here is the determination of the integer N. This issue is commonly referred to in the literature as the integer ambiguity problem.
In a real world situation where the satellite (and possibly the observer) is moving and there is noise and all sorts of distortions are present, the issue of integer ambiguity in the prior art is tackled in two steps. An initial acquisition procedure to determine N is followed by a carrier phase tracking loop arrangement to keep the correct integer up to date.
With the above as background it is now possible to give a meaningful list of problems/issues that have occupied the GPS interferometry community over the last 20 years or more and whose proposed solutions are the subject matter of all patents in this area.
Issues and Problems
1. Loop Settling Times
It is clearly highly desirable to perform the above calculations as soon as possible in most cases. To that end a lot of work has been done in balancing settling times and noise contributions.
2. Effective Noise Suppression
Noise suppression in the prior art has been addressed by resorting to optimal estimation methods. These include Kalman filtering, Maximum Likelihood Estimation and many others. These methods can get quite involved and they can also have a negative effect on settling times. Kalman filtering, for example, is essentially a mathematical embodiment of a tracking loop. A measured output is compared with an expected output based on the available knowledge of the noise statistics. The difference between the two is used to better predict the next expected output. The adequacy of this process hinges crucially on the correct knowledge of the noise statistics. These can be deduced from measurements but the estimation accuracy is proportional to the amount of data considered, which in turn means the longer one waits the better the results.
3. Integer Ambiguity.
This calculation can get very involved using measurement differences, double differences, Kalman filtering and Diophantine equations (equations with only integer solutions). Once established, the correct value of N must be maintained. This usually entails the use of a PLL which can also get complicated in that it must not only correctly measure phase but also take into account any possible ‘cycle slipping’. A cycle slip is a common occurrence in phase locked loops and is frequently not an issue. Here a cycle slip means one, or several cycles of phase difference before and after the slip, in other words a possible change in N by some integers, an unacceptable situation. A lot of work has been done in this area in the prior art.
4. Phase Ambiguity.
This is another ambiguity, which comes about because of the way phase angles must be processed. Whenever mathematical operations involving phase angles have to be performed it is essential to use modulo 360° arithmetic. This means that if, for example, the true phase output is 460° it will register only as a 100° phase output, giving a wrong answer. The resolution of such ambiguities, especially for very large possible true phase values, can get extremely involved.
5. Range Accuracy
As illustrated above, the range accuracy required by interferometry is in the millimeter range or better. Besides the obvious effects of noise and its suppression, which has already been described above, there is another issue which may become relevant: temperature induced changes. Especially if the antennae are mounted on metal platforms, temperature induced expansion and contraction may well be at the millimeter level or higher. Some means of compensation may therefore become necessary.
6. Differential Delays
At the frequencies of interest (1575.42 MHz) a delay error of 1 picosecond is equivalent to a phase error of 0.567°. Equivalently, a 1 millimeter difference in two cables will produce a phase difference of 1.89°. These are very small differences, producing relatively large effects. Great care must be taken to deal with these issues.
7. Cost
Conventional designs based on the principles outlined above, will tend to be costly, both in materials and maintenance. To reduce costs the drivers should be simplicity and robustness of design and reliance on proven, well understood sub-systems and processing methods.
Requirements of the Present Invention
With all of the above as background it is now possible to discuss the novel features claimed in the proposed patent.
As stated above, the military have a requirement for very accurate pointing of an antenna, whilst moving in a ground vehicle over rough terrain. The issue whether acceptable solutions to problems of this nature exist, hinge totally on the specific requirements that must be met.
Road tests conducted by the military have established worst case angular rotations for a moving vehicle about three orthogonal axes. Assuming the x-axis to be in the direction of the vehicles motion, the y-axis at right angles to that and the z-axis in the up-down direction, then the worst case changes about the x, y and z axes per second were found to be 15°, 60° and 30° respectively. This implies that the slowest update rate must be in excess of 100 times a second to permit a worst case change (about the y-axis) of 0.5°. So a receiver producing an attitude estimate to about 0.1° every 5 msecs would be needed. An inexpensive receiver meeting these requirements is the subject of this invention. No such solution appears to be available in the prior art.
An Attitude System for COTM
It has become clear that, if a GPS based solution meeting the above requirements is possible, all conventional GPS receiver designs are unacceptable. Closed feedback tracking loops, Kalman filtering or other sophisticated noise mitigation processing, complex integer ambiguity reduction processing, must be dispensed with if the solution update rate and low cost goals are to be met. It will be demonstrated that the proposed system solves/satisfies all of the problems/issues listed above.
The system to be described here is a novel design with little in common with GPS receivers as they are known today, although its sub-systems are made up of conventional parts and use conventional techniques. Nevertheless the combination of these conventional parts and techniques produce an end product which is both novel and non-obvious, and which will meet specifications unreachable by any other GPS based attitude system currently described, known or available in the prior art.